Generally, a block pattern is employed as the tread pattern of a winter tire (a studless tire). The block pattern enhances on-snow performance by facilitating biting of blocks into snow which is to be fitted into a space between blocks adjacent to each other in the circumferential direction of the tire. In addition, braking and driving performance on an icy road surface (on-ice performance) is enhanced by a water-film removing effect and an edge effect which are achieved by providing to the blocks sipes extending in the width direction of the tire.
Of these, the on-ice performance can be further enhanced simply by increasing the number of sipes in the circumferential direction. However, the increase in the number of the sipes results in short intervals of sipes, so that the block rigidity is lowered, and the amount of deformation of the blocks is increased. For this reason, stress is excessively concentrated on the bottom portions of the sipes, so that cracks are more likely to be formed in the bottom portions of the sipes. In other words, when the number of sipes is simply increased to raise the on-ice performance, and thus the intervals of the sipes are shortened, the durability of the tire is degraded. In this respect, for a method for raising the on-ice performance of a tire without impairing the durability thereof, it is preferable not to shorten the intervals of the sipes.
For example, Japanese patent application Kokai publication No. 2001-39126 describes that, in addition to sipes extending in the width direction of a tire, other sipes having a shallower depth are provided in such a manner as to cross the sipes extending in the width direction of the tire. Also Japanese patent application Kokai publication No. 2003-237320describes that deformation of small blocks defined by open sipes provided to open into main grooves is suppressed by causing the heights of the small blocks to be different between adjacent small blocks, so that the sipe depths can be offset alternately.
However, in the former case, since the blocks become smaller by the provision of the crossing sipes, the effect of enhancing the durability can not necessarily be said to be large. Meanwhile, in the latter case, although some measures are taken for the durability, blocks having different heights are formed. Hence, there is a problem that blocks having a larger height have a higher contact pressure, and thus are more worn. This also can not necessarily be said to achieve sufficient durability.